I've been with Smsrty for about a year now. I've had no problems at all, with no evidence of slow speeds or network congestion in that time. I churned off ID Mobile as they were incompetent and it took several contacts to fix the problem they caused (and I never did get the inconvenience compensation they offered).
Something else to consider is unlike Smarty, ID Mobile log a hard search on your credit file, even just for a £4/month 30 day contract, which is unnecessary, and something you might want to avoid if you're applying for credit for other reasons.
I've been with Three and have had Smarty for the last couple of years. I can't notice any functional difference between them. I've never had need of customer service, so can't speak to that. With Smarty you pay less than Three, still get the free roaming (up to 12GB), and now can even have an eSIM.
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Something else to consider is unlike Smarty, ID Mobile log a hard search on your credit file, even just for a £4/month 30 day contract, which is unnecessary, and something you might want to avoid if you're applying for credit for other reasons.